This invention relates to a projection exposure apparatus and, more particularly, to a projection exposure apparatus usable in the manufacture of semiconductor devices such as integrated circuits and for projecting a pattern of a photomask or reticle onto a semiconductor wafer through a projection optical system including a plurality of refracting surfaces.
Projection exposure apparatuses are well known in the field of manufacturing semiconductor devices. In such projection exposure apparatuses, a circuit pattern formed on a photomask or reticle (which hereinafter will be referred to simply as "mask") is projected onto a semiconductor wafer at a real magnification or a reduced scale through a projection lens system whose aberrations are corrected with respect to a predetermined wavelength of light. By irradiating the mask with the predetermined wavelength of light, the wafer is exposed to the pattern of the mask. As is well known in the art, each of the semiconductor devices is manufactured by superposing exposures of the same wafer to different patterns formed on separate masks. In the projection exposure apparatuses, therefore, it is necessary to preparatively detect, prior to each exposure, the positional relation between the pattern or patterns which have already been printed on the wafer and the pattern of the mask which is going to be projected by the projection lens system, and, if the positional relation therebetween is out of a predetermined state, then it is necessary to correct the positional error. The positional relation to be detected includes a relation between the mask pattern and the wafer surface in a direction along the optical axis of the projection lens (which relation will hereinafter be called "focus relation") and a relation, in a plane perpendicular to the optical axis of the projection lens, between the mask pattern and the pattern or patterns which have already been printed on the wafer (which relation will hereinafter be called "alignment relation").
From viewpoints of accuracies of detection and reduction in time of detection, it is preferable to detect such positional relation through the projection lens system, which is called a through-the-lens (TTL) system. However, since, a refraction optical system such as the projection lens system shows different imaging performances for different wavelengths of light, as compared with a reflection optical system, it involves difficulities to effect the above-described detection by means of the projection lens whose aberrations have been corrected to provide optimum imaging performances only relative to the wavelength of light exposing the wafer to the mask pattern. A proposal has been made in IBM Technical Disclosure Bulletin Vol. 18, No. 2, pp. 385 and 386 published July 1975. According to this proposal, the same wavelength of light is used for the exposure and for the detection of the positional relation. This may be preferable because of its simplicity. However, in some cases of recent semiconductor device manufacturing processes, the wafer surface is subjected to an anti-reflection treatment with respect to the exposure light or an absorption promoting agent is added to or incorporated into a sensitizing agent, in order to obtain better printing characteristics. In these cases, no reflection light or only little reflection light is obtainable from the wafer surface. As the result, the detection of the positional relation would be very difficult to execute, in accordance with the above proposal.
Other proposals, such as the following, have been made, directed at the detection of the positional relation with a wavelength of light other than that for the exposure.
Proposal A: To use a projection lens whose aberrations have been corrected relative to both of two different wavelengths of the exposure light and the detection light.
Proposal B: To use an additional optical system such as a lens, a parallel flat plate, a mirror, etc. which is provided separately from the projection lens and is disposed outside the path of the exposure light. This additional optical system is employed to change the optical path length for the detection light. Examples are disclosed in U.S. Pat. Nos. 4,357,100 and 4,492,459 issued Nov. 2, 1982 and Jan. 8, 1985, respectively.
Proposal C: To replace a part of the plural lens components of the projection lens system arranged for the exposure, by an additional element or elements for the sake of detection of the positional relation. An example is disclosed in U.S. Pat. No. 3,897,138 issued July 29, 1975.
Proposal D: To change, upon detection, the position of the wafer in the direction of the optical axis of the projection lens, as compared with the position of the wafer, upon its exposure. An example is disclosed in Japanese Patent Application Laid-Open May 25, 1984, Laid-Open No. 90929/1984.
Proposal E: To provide, on the wafer surface, a mark such as a Fresnel mark for converging the detection light toward a predetermined point. On the basis of the thus converged light, the positional relation is detected.
With these proposals, however, there still remain the various following inconveniences.
As regards the proposal A, it involves difficulties to make such a lens system which is corrected relative to both of the two wavelengths. Second, as the difference between the two wavelengths gets larger, there occurs a larger amount of focus displacement depending on variations in the wavelength, particularly in a range near the exact wavelength for the exposure. In other words, the depth of focus becomes smaller. As the result, it is very difficult to assure sufficient printing performances if the exposure light is not provided by a beam having an absolutely single wavelength, such as a laser beam, that is, if the exposure light is provided by a beam having a spectrum range.
As regards the proposal B, if the additional optical system is fixed, it limits the positions of detection marks formed on the mask and wafer for the sake of detection of the positional relation (particularly the alignment relation). Also, it adversely affects the size and/or shape of the pattern (actual device pattern) to be formed on the mask. If, on the other hand, the additional optical system is made retractable to a position which does not adversely affect the exposure light upon exposure, substantial time is required for such retracting movement. Further, a complicated mechanism is required to maintain the positional accuracies of the movable additional optical system.
With regard to the proposal C, additional time is necessary for the movement or replacement of the lens components. Also, it involves a problem of positional accuracies for the replaceable lens elements.
With respect to the proposal D, additional time is required for the movement of the wafer. Further, the following problem occurs. That is, if the projection lens system has been corrected relative to the exposure wavelength only, and when such a wavelength of light other than the exposure wavelength is incident on the projection lens system, the image formed in an image plane defined by the second-mentioned wavelength other than the exposure wavelength is not so reliable. This is because the aberrations of the projection lens system have not been corrected relative to the second-mentioned wavelength. If, therefore, the mask is provided with a detection mark for the sake of detection of the positional relation (particularly the alignment relation), an exactly correct image of the detection mark is not obtainable of the wafer (even if the wafer is displaced to an in-focus position). This leads to a possibility of failure of accurate detection of the positional relation.
As regards the proposal E, a complicated mark has to be formed on the wafer. Therefore, it is not advantageously applicable to step-and-repeat reduction exposure apparatuses which are usable with such a wafer having actual device patterns each formed with plural detection marks.